This invention relates to gas separation. In particular, it relates to a vacuum swing adsorption apparatus for separating a gaseous mixture, for example air.
There is a continuing search for more economical methods of separating air. Some of these methods are directed at improving the thermodynamic efficiency of air separation processes. Others are directed at finding inexpensive sources of power. One such source is the waste heat that is typically evolved from an oxidation reaction. It is known to employ such waste heat to raise steam and to expand the pressurized steam in a turbine which may be directly coupled to a compressor forming part of the air separation plant or which may alternatively be coupled to a generator of electrical power. Alternatively, a compressed nitrogen stream from the air separation plant may be raised in temperature by the waste heat and expanded in an expansion turbine. The expansion turbine may be directly coupled to a compressor forming part of the air separation plant or to a generator of electrical power.
One disadvantage of such arrangements is that they add very considerably to the total capital cost of the plant and they tend not to lend themselves for use when there is a need to vary the output of electrical power.
Japanese patent application 54-162697A discloses a process in which an oxygen-enriched gas containing 40% by volume is separated by means of membranes from air. The oxygen-enriched gas is the permeate gas. A vacuum is drawn on the permeate gas side of the membranes in order to facilitate the separation of the oxygen-enriched gas. The oxygen-enriched gas is supplied to a furnace. Waste heat from the furnace is employed in raising steam. The steam flows through an eductor which draws the vacuum. Such an arrangement is however unsuitable for practical use because the steam contaminates the oxygen-enriched product. It is therefore necessary to disengage liquid water from the resulting oxygen-enriched gas. It is also necessary to ensure that the steam contains no impurity that might be hazardous in the presence of the oxygen-enriched gas.
It is well known to separate air by vacuum swing adsorption. A motor driven vacuum pump such as a Rootes blower is used to draw the vacuum. Such vacuum pumps are prone to failure. Further, vacuum swing adsorption apparatus including such a vacuum pump tends to have relatively poor turndown characteristics and is therefore not suitable for use in meeting a variable demand for a gaseous product.
It is an aim of the present invention to provide an apparatus which ameliorates at least one and preferably all of these disadvantages.